Newton's laws - AQA SynergyKinetic energy

Forces cause changes in the motion of an object. Newton’s Laws help us to understand and analyse those changes. Unbalanced forces cause changes in speed, shape or direction.

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Kinetic energy

Kinetic energy is one type of energy store. It is the energy stored in a moving body. The amount of kinetic energy stored in a body depends on its mass and its speed.

Calculating energy in the kinetic store

The amount of of a moving object can be calculated using the equation:

Kinetic energy = ½ × mass × velocity2

\(\text E_{k} = \frac{1}{2} \text m v^{2}\)

This is when:

  • energy (\(\text E_{k}\)) is measured in joules (J)
  • mass (\(\text{m}\)) is measured in kilograms (kg)
  • velocity (\(\text{v}\)) is measured in metres per second (m/s)

Example

An apple of mass 100 g falls from a tree. It reaches a speed of 6 m/s before landing. What is the gain in kinetic energy of the apple?

\(\text E_{k} = \frac{1}{2} \text m v^{2}\)

\(\text E_{k} = \frac{1}{2} \times 0.1 \times 6^{2}\)

\(\text E_{k} = \frac{1}{2} \times 0.1 \times 36\)

\(\text E_{k} = 1.8 \text J\)

Question

How much kinetic energy does a 30 kg dog have when it runs at 4 m/s?

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